Precise branching ratios to unbound 12C states from 12N and 12B β-decays

Two complementary experimental techniques have been used to extract precise branching ratios to unbound states in 12C from 12N and 12B β-decays. In the first the three α-particles emitted after β-decay are measured in coincidence in separate detectors, while in the second method 12N and 12B are implanted in a detector and the summed energy of the three α-particles is measured directly. For the narrow states at 7.654 MeV (0+) and 12.71 MeV (1+) the resulting branching ratios are both smaller than previous measurements by a factor of ≃2. The experimental results are compared to no-core shell model calculations with realistic interactions from chiral perturbation theory, and inclusion of three-nucleon forces is found to give improved agreement.